This invention relates generally to an electrostatographic printing machine, and more particularly concerns improved transfer and fusing systems for use therein.
In the process of electrostatographic printing, an electrostatic latent charge pattern is reproduced in viewable form. The field of electrostatography includes electrophotography and electrography. Electrophotography employs a photosensitive medium to form, with the aid of electromagnetic radiation, an electrostatic latent charge pattern. Electrography utilizes an insulating medium to form without the aid of electromagnetic radiation, the electrostatic latent charge pattern. In both of the foregoing processes, the electrostatic latent image is developed with toner particles which are ultimately transferred to a sheet of support material. Hereinafter, an electrophotographic printing machine will be described as an illustrative embodiment of the foregoing process. This printing machine incorporates the features of the present invention therein.
In the process of electrophotographic printing, for example, as disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in l942, an image bearing member or photosensitive element having a pnotoconductive insulating layer is charged to a substantially uniform potential to sensitize the surface thereof. Subsequently, the charged photoconductive surface is exposed to a light image of the original document. The light image selectively dissipates the charge in the irradiated areas of the photoconductive surface in accordance with the light intensity projected thereon. This creates an electrostatic latent image on the photoconductive surface. Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing a developer mix into contact therewith. Typical developer mixes comprise dyed or colored thermoplastic particles, known in the art as toner particles, which are mixed with coarser carrier beads, such as ferromagnetic granules. The developer mix is selected such that the toner particles have the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface.
In multi-color electrophotographic printing, the light image employed to irradiate the charged photoconductive surface is filtered to form a single color electrostatic latent image thereon. Each single color electrostatic latent image is developed with toner particles of a color complementary to the color of the filtered light image. Thereafter, the different color toner powder images formed on the photoconductive surface are transferred to the sheet of support material in superimposed registration with one another. Generally, the multilayered toner powder image is permanently affixed to the sheet of support material forming a color copy thereon. The foregoing process is described generally in U.S. Pat. No. 3,799,668 issued to McVeigh in l974. In this type of process, the sheet of support material is secured onto an electrically biased roll which rotates in synchronism with the photoconductive drum. The toner powder images developed on the photoconductive drum are transferred to the sheet of support material secured to the electrically biased transfer roll. Generally three toner powder images are transferred, in superimposed registration with one another, from the photoconductive drum to the sheet of support material. The sheet of support material is usually secured by mechanical gripping means to the electrically biased roll. This gripping apparatus frequently does not permit copying to the edges of the sheet of support material.
In addition, this degradates the quality of duplex copies. More particularly, in duplex printing a toner powder image is transferred to the side of the sheet opposed from the first copy. The fused toner powder image of the first copy acts as a dielectric layer reducing the transfer fields and the efficiency of the transfer process. Thus, the toner powder images may not be totally transferred to the side opposed from the first copy resulting in low quality duplex copy.
Various systems have been proposed to overcome this problem. By way of example, an intermediate roller may be employed. In devices of this nature, successive toner powder images are transferred, in superimposed registration with one another, from the photoconductive drum to an intermediate roller. In many cases, such systems employ three photoconductive drums, one for each of the toner powder images being forwarded, U.S. Pat. No. 3,392,667 issued to Cassell et al. in 1968 and U.S. Pat. No. 3,399.611 issued to Lusher in 1968 describe printing machines of this type. However, systems of this type have other problems. For example, the transfer efficiency of the multi-layered toner powder image in contact with the intermediate roller is low. This frequently produces unacceptable color shifts in the final fused powder image on the sheet of support material. In addition, when the multilayered toner powder image is fused to the sheet of support material the power requirements are significant. For example, approximately 60% to 80% of the power employed in the powder image is used to heat the sheet of support material. Moreover, when the sheet of support material is a transparency, hollow characters frequently result due to inefficiencies in the transfer apparatus. Nor does this solve the duplexing problem.
Accordingly, it is the primary object of the present invention to improve color electrostatographic printing machines decreasing the fuser power requirements and improving duplexing capabilities.